A conventional discharge lamp lighting apparatus will be described with reference to drawings. FIG. 3 is a diagram showing a construction of a discharge lamp lighting apparatus of a lamp electric power feedback method which has conventionally been known.
In FIG. 3, reference numeral 1 denotes a DC source; 2 an inverting circuit having switching elements such as MOSFETs; 3 a driver; 4 a load circuit; 5a to 5c lamps; 6 a coupling capacitor; 7a to 7c ballast coils; and 8a to 8c starting capacitors.
In FIG. 3, reference numeral 9 denotes a load current detecting resistor; 10 an integrating circuit; 11 an operational amplifier; 11a a target value; further, 13 a control circuit; 14 a latch circuit; 19a to 19c lamp voltage detecting circuit; 20 an overvoltage detecting comparator; and 20a a threshold value.
The operation of the conventional discharge lamp lighting apparatus will be mentioned hereinafter.
The inverting circuit 2 is driven by the driver 3 and the DC source 1 is converted to a high frequency output. The high frequency output from the inverting circuit 2 lights the lamps 5a to 5c through the coupling capacitor 6 and ballast coils 7a to 7c.
An active component (effective value) of the load current of the load circuit 4 is detected by integrating a voltage generated in the detecting resistor 9 by the integrating circuit 10 and set as a negative input of the operational amplifier 11. The operational amplifier 11 sets the target value 11a as a positive input. An error-amplified output is outputted to the driver 3. By the feedback of the operational amplifier 11, the driver 3 controls a switching frequency of the inverting circuit 2 so as to hold the effective value of the load current of the load circuit 4 constant.
Thus, the consuming electric power of the lamp is held constant. Specifically speaking, as shown in FIG. 4, a feedback is performed so that an active component of drain currents of the switching elements constructing the inverting circuit 2 is constant and the frequency is controlled.
Since a lamp voltage increases and the feedback is performed in the direction where the lamp current is suppressed if the life of the lamp is shortened, the frequency beams high and the drain current waveform comes into what as shown in FIG. 5.
In this case, the active component of the drain current is held constant due to the increase in frequency. However, a reactive current also increases simultaneously.
In the case where the life state of the lamp is further shortened and it becomes the life ending, if the frequency is high, the reactive current component further increases. When the reactive current exceeds a certain degree, the switching elements are heated, so that it causes a trouble.
Protecting means for the life ending lamp will be mentioned hereinbelow. The lamp voltage of the life ending lamp is higher than that of the normal lamp. As shown in FIG. 3, the lamp voltage is detected by lamp voltage detecting circuits 19a, 19b, and 19c. The outputs of those detecting circuits are inputted to the comparator 20. In the case where the detection value exceeds a predetermined threshold value 20a, the driver 3 is controlled by the operation of the control circuit 13 so that the inverting circuit 2 is protected. Such means has been known.
As for a protecting method, there are a method of stopping an oscillation of the driver 3, a method of forcedly increasing an oscillation frequency of the driver 3 and decreasing both of an active component and a reactive component of the drain current (hereinafter, referred to as a "protection mode operation") and the like. It is latched by the latch circuit 14.
The operation of the lamp voltage detecting circuits 19a (similarly with regard to 19b and 19c) is performed in such a manner that the lamp voltage is divided by capacitors 21 and 22, rectified by diodes 23 and 24, and integrated by a resistor 25 and capacitor 26, thereby obtaining a detection value.
As a result, if one lamp or more in the life ending state exist among the three lamps, the inverting circuit 2 is protected by the operation of the control circuit 13.
However, in the foregoing conventional discharge lamp lighting apparatus, the lamp life ending detection has to be executed every lamp. Particularly, with regard to the apparatus having many lamps, it is a problem that the number of parts increases and costs increase.
In the case where a plurality of (for instance, three) lamps which are not detected in the life ending detection but the life state is slightly shortened (hereinbelow, referred to as a "half-life ending lamp") are connected, a feedback is performed so that lamp currents are set to be constant in total. Consequently, the frequency is made high similar to the case where (for example, one) life ending lamp is mounted, and the burden of the inverting circuit is increased by the increase in the reactive current of the drain current, which causes another problem that the voltage that indicates life ending is not detected in any of the lamp voltage detecting circuit 19a to 19c to continue an ordinary operation despite of the operation state that is properly meant to be protected.
This invention is accomplished to solve the above mentioned problems. It is an object of the invention to provide a discharge lamp lighting apparatus in which the circuit can be protected from a life ending lamp without executing a life ending detection for each of lamps even if a plurality of lamps are mounted and the burden of the inverting circuit can certainly be detected and protected by detecting in total the states of lives of the plurality of lamps.
It is an object to provide a discharge lamp lighting apparatus in which in the case where the lamp is in the life ending state or the like, the protection mode operation can be performed with a simple construction.